This invention is primarily applicable to the recovery by flotation of gold from ores and metallurgical plant products such as the treatment of a gold ore in a metallurgical circuit following gravity concentration. It is further applicable to the recovery of gold values remaining in the tailings after treatment by cyanidation. Further, where valuable sulphides are present in such ores such as pyrite, which may be further processed for the production of sulphuric acid and in addition, the pyrite may carry substantial gold bearing sulphide, or complex mixtures of sulphides such as arseno pyrite, which may be a gold bearing mineral, silver sulphides or complex sulphide minerals of bismuth, all such minerals together with the gold values may be floated as a single bulk concentrate with surprisingly high recovery of all of such minerals present. In certain materials, which in addition to gold contain uranium values a surprisingly high percentage of the uranium minerals are also recovered in the same bulk flotation concentrate.
In treating simple metallurgical gold bearing ores the cyanide process has been the conventional practice for many years. Recovery of the gold values in using such a process may vary from a low of about 90% to a high of about 98% of the contained gold. Such ores and treatment are typical of the deposits in the Republic of South Africa wich produces by far the bulk of the world output of gold. Heretofore the major part of gold values in the tailings from these plants was thought to be contained in totally locked particles where the cyanide was unable to attack the gold particles and thus were unavailable for leaching by the cyanide process.
In carrying out research on these ores with my flotation process I found that this theory was wrong as in a high grade flotation concentrate I was able to recover in excess of 75% of the total gold values remaining in such plant tailings following cyanidation. Further, in using a sulfhydryl anionic collector as the only collector, up to 50% of the uranium values occurring in tailing floated in the same concentrate with the gold. A further surprising feature was the very high recovery of the pyrite. Cyanide is known as one of the most effective depressants for pyrite and in applying my process to old plant tailings, which in addition to having been treated by the cyanide process were also oxidized through years of storage in a tailings disposal area, the pyrite recovery which in this case has a major economic value for sulphuric acid production, readily floated in the same circuit and recoveries were in excess of 90% of the contained pyrite in the tailings.
In applying my invention to ores, or metallurgical plant products, I use at least one sulfhydryl anionic collector. On ores or metallurgical plant products containing metallic oxides, to improve their recovery, I prefer to use at least one sulfhydryl anionic collector in combination with at least one other collector selected from the group consisting of oxyhydryl collectors and cationic collectors. The sulfhydryl anionic collectors, oxyhydryl anionic collectors and cationic collectors are well known in the art and classified in "Flotation" Second Edition, A.M. Gaudin, McGraw Book Company, Inc., New York, 1957, pages 184-186.
The term "Agitation Conditioning Stage" normally consists of a multiplicity of agitators. In my preferred circuit I prefer to use a minimum of two agitation conditioners in any single conditioning stage. The usual distinguishing feature between each agitation conditioning stage is where I either add an acid agent or an alkaline agent to change the pH range within the individual agitation conditioning stage. Alternately, where I add an acid agent to lower the pH of the pulp within the pH range of about 1.5 to about 5.0 the initial pH may be at 1.5 and due to the acid consuming constituents in the material being treated may rise to as high as a pH of 7 at the end of this acid conditioning stage. Where my next stage is in the pH range of about 6.0 to 11.0 I may not add an alkaline agent to change the pH, but at the end pH of the acid stage add a collector such as potassium amyl xanthate and condition for a sufficiently long period and with sufficient power input to the pulp to heavily activate the desired recoverable minerals. In this case, at the beginning of the second stage the pH will be 7.0 and the pH may be slightly higher or lower at the end of the second stage say within the pH range of 6.5 to 7.5 prior to the pulp being fed to the flotation circuit with the final addition of a suitable frother.
By "Acid Agent" I mean at least one agent selected from the group consisting of sulphuric acid, sulphurous acid and sulphur dioxide, and is used to lower the pH in the alkaline pH range or reduce the pH of the pulp to a desired point or range in the acid pH range of about 1.5 to about 5.0. My preferred acid agent is sulphuric acid.
By "Alkaline Agent" I mean an agent selected from the group consisting of lime, calcium hydroxide, sodium carbonate, potassium carbonate, sodium hydroxide and ammonium hydroxide, and is used for upward adjustment of the pH of the pulp.
When I use the term "pounds per ton" of various reagents, this is pounds per metric ton of the total original feed to my circuit, unless otherwise specified.
In describing the practice of my invention the term "agitation conditioning" as distinguished from conventional practice is important.
In mixing reagents with the pulp to procure collector coating of sulphide minerals conventional practice uses the lowest possible agitation conditioning speeds with the main purpose being to keep the solids in the pulp in suspension and distribute the reagents throughout the pulp. To quote Taggart, "Handbook of Mineral Dressing", John Wiley & Sons Inc., New York, March 1947, Section 12, page 20: "With fine pulps large tanks and slow agitation as by slow sweeps, will serve, the principal consideration in this case being dispersion of the reagents."
Conversely, in my agitation conditioning stages I use vigorous to violent agitation with substantially higher power to the agitator mechanism or mechanisms than would be required to keep a finely ground product of the ore in suspension at a specific pulp density. The power input to comparable sized agitators in the practice of my invention is normally at least twice the amount of power that would be required alone to keep the solids in the pulp in suspension.
The agitation conditioning times are important. In my at least one acid conditioning stage in the pH range of about 1.5 to 5.0 to obtain optimum results, the minimum agitation conditioning time is about 6 minutes and the maximum about 30 minutes. The optimum range is normally in the time period of about 9 minutes to about 18 minutes. In my at least one additional agitation conditioning stage in the pH range of about 6.0 to 11.0 the minimum period of time is about 3 minutes and the maximum about 20 minutes.
Where I refer to an "optimum pH point", I mean the practical pH point at which the pulp can be maintained. For instance, if I refer to an optimum pH point of 7.5 in an alkaline agitation conditioning stage, in plant practice it may vary plus or minus approximately 0.2 with erratics due to changes in plant operating conditions or poor operating plant control.
When I refer to "optimum pH point" for instance in an acid conditioning stage it may be a range of pH's particularly if the ore consumes a high percentage of the acid agent fed to it. For instance on the addition of sulphuric acid to the first agitator in a three step (three agitators) agitation conditioning stage, the pH of the pulp may drop to as low as 1.5, and then at the end of say the third agitator which would represent a total conditioning time of 15 minutes, the pH of the pulp will have risen to say about 5.0. In this case, the optimum pH point would be at a designated recording point in the first agitator, or at its discharge point.
In describing my invention the expression "suitably prepared pulp" of an ore or material, when used herein is intended to mean that the pulp has been made up from a material that has been ground to flotation feed size for reasonable liberation of the desired mineral constituents and has during such comminution or thereafter been subjected to such treatment steps (such as adjustment of pulp density by dilution, thickening, thickening and dilution) as the operator may deem appropriate in the case of the particular material being treated, to present the pulp for the agitation conditioning stages of treatment comprising the process of the present invention.
The principal object of the invention is to provide a process for the economic recovery of gold values by flotation from various types of ores and metallurgical plant products such as the tailings from a cyanidation plant.
It is a further object of the invention to float in the same circuit sulphide minerals, and if present, oxide minerals of uranium.
It is a further object of the invention to produce high grade gold concentrates from gold bearing ores and metallurgical plant products.
It will be appreciated that with the production of high grade gold concentrates such as in the case of from cyanidation plant tailings in the Republic of South Africa, the ratio of concentration will be as high as 80 to 1. Thus, with such a concentrate, which would represent 1.25% of the original tailings and containing in excess of 75% of the gold values and up to 50% of the uranium values together with in excess of 90% of the pyrite values, such a concentrate can be further treated in a number of conventional ways. For instance, the concentrate may be first rotated recovering the sulphur from the pyrite to produce sulphuric acid. Following this step the roasted product can be acid leached with sulphuric acid to recover the uranium values. Following this step the tailings from the acid leaching process can be cyanided to recover the gold values.
It is a further object of the invention to recover minerals other than gold such as the platinum group minerals from both ores and metallurgical plant products.